primary c's a small number of fundamental colors. In visual science this refers to red, green, and blue, the colors specifically picked up by the retinal
cones; mixtures of varying proportions of the primary colors will yield the 150 discriminable hues of normal human vision. In painting and printing, the primary colors are red, blue, and yellow.
color vision deficiency inability to distinguish between certain colors, popularly called “
color blindness.” A complete deficiency (
monochromatic vision), the total inability to see colors, is rare, affecting only one person in 300,000. Much more common are the various types of partial deficiency. The most common is red-green confusion (see
deuteranopia and
protanopia), which affects approximately 8 million people in the United States.
Color vision is a function of the
cones in the retina of the eye, which are stimulated by light and transmit impulses to the brain. It is now thought that there are three types of cones, each type stimulated by one of the primary colors in light (red, green, and blue or violet). In red-green color vision deficiency, there is a deficiency of either red or green receptors, so that the two colors do not appear distinct from each other.
Color vision is usually tested by the
ishihara test with a series of
pseudoisochromatic charts or plates. (See Atlas 4, Part F.) These have a letter, number, or symbol printed in dots of one color in the midst of dots of gray or other colors. The normal person can see the symbol with no difficulty, but the person with color vision deficiency cannot distinguish it from the background.
Although color vision deficiency may occasionally result from injuries, diseases, or certain drugs, most cases are hereditary. It is usually inherited by males through the mother, who carries the trait from her father although she is not color deficient herself. In some cases, if the grandfather is color deficient and the mother carries the trait, a daughter may inherit color vision deficiency, but the ratio of men to women with inherited forms is about 20 to 1. There is no known cure for color vision deficiency.